Physics Essay 1.8 updated Fall 2011

The Eye

What are the parts of the eye? How does the spatial arrangement of these parts affect your vision and color perception?

The diagram of the eye below shows the important parts of the eye. The part of the eye that registers light from the outside world is called the retina and is shown in blue. The retina covers the entire inside-back portion of the eye. Rod cells and cone cells are embedded in, and distributed across, the retina and these cells are the structures that actually register light. When light hits these cells, the cells send a signal to the brain that light is striking a certain part of the retina. In this way, the retina acts a bit like a screen and when the brain receives electrical signals from the rod and cone cells, the brain knows what kind of light is hitting different parts of the retina.

Rod cells cannot distinguish between different colors of light. These rod cells can only register the intensity or brightness of the light that strikes them. It is the cone cells that can register color. There are three different kinds of cone cells: those that register red light, those that register green light, and those that register blue light. Thus, when bright light strikes a rod cell, the rod cell sends an electrical signal to the brain and the brain knows that bright light is hitting that portion of the retina. If this bright light is blue, then the cone cell right next to the rod cell sends a signal to the brain that the light striking this potion of the retina is blue. With these two pieces of information, the brain knows that bright, blue light is striking that portion of the retina.

Figure 1, This illustration of the eye shows the most important parts involved in forming images and seeing color

The optic nerve carries the electrical signals to the brain, while the iris changes size to let more or less light into the eye, through the hole in the middle of the iris, also known as the pupil. The lens of the eye can refract incoming light to help form clear images. We will experiment with lenses after we experiment with optical illusions and mirrors. The eye is filled with a liquid like vitreous gel.

The rod and cone cells can be referred to as photochemical receptors. The three different types of cone cells (red receptors, blue receptors, and green receptors) can work in concert to help perceive colors such as yellow, magenta, and cyan. When red and blue light strikes a certain portion of the retina the red-reading cone cells start working and sending signals to the brain. The blue-reading cone cells in this region do the same, but the green-reading cone cells in this region remain ‘off’ and send no signal to the brain. Thus, the brain ‘sees’ magenta in this region.

The nerve pathways that carry the signals to the brain are made of individual nerve cells. When triggered, each nerve cell releases a chemical that travels across a gap (the synapse) to the next nerve cell. This chemical then triggers the next nerve cell in the pathway, allowing the signal to move along the pathway to the brain. Other chemicals are then released to ‘neutralize’ the initial chemical and end the signal, but this process can take time. Cones that have been subjected to a certain kind of light can become overworked and over time send weaker signals to the brain.

Your job in the Optical Illusion Lab is to combine this new knowledge about the eye with what you know about reflection, refraction, and light/color mixing and explain how the different illusions are created.